We have developed fluorescent nanoparticles (NP) conjugated to the anticancer natural product geldanamycin (GA) to be used initially as a molecular probe for studying the pharmacological effects of GA in live cancer cells. GA is linked to the nanoparticle via a polymer coating that a) improves mechanical and chemical stability of the fluorescent nanocrystal core, b) controls the drug loading on the surface of the nanoparticle, and c) allows unassisted intracellular delivery of fluorescent nanoparticles to tumor cells. By controlling the surface loading of the drug (from 5 to 300 molecules) we have been able to demonstrate a dose response for both drug uptake and cytotoxicity in cultured cancer cells. Geldanamycin-sensitivity in cancer cells is dependent upon two different mechanisms, Hsp90 specific recognition as well as an active uptake pathway. These two factors make geldanamycin an interesting selective probe for directing nanoparticle uptake in animal tumors. This proposal aims to define the GA-dependent cellular uptake and tumor selectivity of nanoparticles that can be used as new imaging probes or multivalent drug delivery particles. [unreadable] [unreadable] [unreadable]